1. Field of Invention
The present invention relates to a thin film capacitor formed on an insulating board. In particular, it relates to a thin film capacitor that is suitably mounted in an electronic circuit board made for example, but not by limitation, one of an organic material, a method for manufacturing the same, and a printed circuit board incorporating the thin film capacitor.
2. Description of the Related Art
Recently, there has been an increased market demand for a higher packaging density of passive components for a higher performance of electronic equipment. To meet such a demand, passive components have been downsized. For example, 1005-size passive components (L: 1.0 mm by W: 0.5 mm) were developed, and then 0603-size passive components (L: 0.6 mm by W: 0.3 mm) were developed. Furthermore, 0402-size passive components (L: 0.4 mm by W: 0.2 mm) are now being developed. On the other hand, there is a perception that further downsizing of such components is difficult technically. Under such circumstances, a technique of incorporating passive components into an electric circuit board to reduce the required area of the board is in the spotlight. In particular, since capacitors are used in the largest numbers among components of an electronic circuit, the area of the electronic circuit board can be effectively reduced if the capacitors are incorporated in the board.
Techniques of incorporating a capacitor into an electronic circuit board include a technique of inserting an electronic component in a cavity formed in a layer in a multilayer board, such as those disclosed in Japanese Patent Application 2000-243873A and Japanese Patent Application 11-45955A, and a technique of using an insulating layer in a multilayer board as a dielectric layer of a capacitor, such as those disclosed in Japanese Patent Application 2000-277922A and Japanese Application 2001-77539A. For the technique of using an insulating layer for a capacitor, in particular, a technique of making the insulating layer of a resin having a non-organic filler mixed therein to increase the relative dielectric constant thereof has been proposed.
However, in the case of inserting an electronic component in a cavity formed in a layer in a multilayer board, if the electronic component is a typical chip type component with a minimum thickness thereof at 0.3 mm, and is a minimum thickness of a prepreg used is 0.1 mm, the thickness of the board cannot be reduced to 0.5 mm or less even if one layer of the prepreg of the printed circuit board is overlaid on each of upper and lower sides of the component. On the other hand, in the case of using an insulating layer for a capacitor, the relative dielectric constant thereof is 2 to 4, and it is extremely difficult to increase the relative dielectric constant to 30 or more even if a non-organic filler is mixed therein. In addition, since the layer has a thickness of several tens xcexcm at the minimum, the capacitance thereof can be only 9 pF at the maximum (it is assumed that the thickness of the insulating layer is 20 xcexcm and the relative dielectric constant thereof is 20).
To eliminate the disadvantage described above, a thin film capacitor, which is thinner and has a higher capacitance per unit area than the chip type electronic component, has been prepared. There have been proposed a large number of techniques relating to the thin film capacitor, and many of them relate to a thin film capacitor formed on a rigid Si substrate or ceramic substrate. The thin film capacitor formed on the rigid substrate has an advantage that a forming temperature of a dielectric thin film thereof can be 500 to 600xc2x0 C. or more, and thus, a dielectric material having a high relative dielectric constant can be used. On the other hand, it has a disadvantage that the thickness of the substrate is typically 0.1 mm or more, and thus, it is difficult to provide a thickness of the multilayer board including such a thin film capacitor, less than 0.5 mm. In addition, in the case where the thin film capacitor formed on the rigid substrate is incorporated in a resin board, such as a multilayer printed circuit board, there is the possibility that the rigid substrate is damaged due to thermocompression bonding in a manufacturing process of the resin board, for example, when manufacturing a multilayer printed circuit board.
To the contrary, a thin film capacitor formed on a flexible substrate, such as a resin substrate and metal foil, has an advantage that the substrate can be thinner and is not damaged due to thermocompression bonding in the manufacturing process of the multilayer printed circuit board. As for the thin film capacitor formed on the flexible substrate, Japanese Published Application 59-135714 A discloses a technique of providing a metal thin film on a flexible film made of an organic polymer and forming a highly dielectric thin film thereon.
Besides, as a method for incorporating a thin film capacitor in an insulating board, such as a printed circuit board, Japanese Published Application 2001-168534 A discloses a technique in which terminal electrodes are formed on both sides of the thin film capacitor on the flexible substrate and the terminal electrodes are connected to an internal electrode or external electrode of the insulating board a penetrating through hole. In this case, there is a problem in that, while a Cu layer formed on an inner surface of the through hole and the terminal electrodes of the thin film capacitor need to be electrically connected to each other, a sufficient reliability of the connection between the Cu layer on the inner surface of the through hole and the electrodes of the thin film capacitor cannot be assured since the lower and upper electrodes formed with a thin film device can have a thickness of 1 xcexcm at the maximum.
As for another technique of the thin film capacitor formed on the flexible substrate Japanese Published Application 2000-357631 A discloses a technique of providing an adhesive film, preferably a metal oxide adhesive film, to make a flexible substrate made of an organic polymer or metal foil firmly adhere to a highly dielectric non-organic film and a metal electrode film in the flexible thin film capacitor.
In order to increase the reliability of the thin film capacitor having the highly dielectric thin film formed on the flexible substrate, such as an organic polymer film or metal foil, it is effective to enhance adhesions between the substrate and the metal film and between the substrate and the highly dielectric thin film as disclosed in Japanese Published Application 2000-357631 A.
However, in this case, the dielectric thin film itself is deformed and damaged because of a stress caused by, in the formation of the dielectric thin film, annealing the same at a temperature higher than the curing temperature of the resin.
Furthermore, when a resistance and an inductor are to be incorporated in a printed circuit board in addition to the thin film capacitor, if the printed circuit board is to incorporate one each of the passive components, that is, one capacitor, one resistor and one inductor, the incorporated passive components need to be at a certain distance from each other. Thus, there remains a problem to be solved about downsizing of the board incorporating the passive components.
It is therefore an object of the present invention to provide a thin film capacitor which can be mounted on a flexible substrate with high reliability. And another object of the present invention is to enable such a thin film capacitor to be mounted in the printed circuit board with high connection reliability.
According to a first aspect of the present invention, a thin film capacitor comprises a substrate having a thickness equal to or more than 2 xcexcm and equal to or less than 100 xcexcm, a lower electrode on the substrate, which includes at least a highly elastic electrode and an anti-oxidation electrode on the highly elastic electrode, a dielectric thin film on the lower electrode, and an upper electrode on the dielectric thin film, wherein the highly elastic electrode is made of a material having a Young""s modulus higher than that of the anti-oxidation electrode.
According to such a highly elastic electrode, a stress which occurs when the dielectric thin film is formed can be reduced. Thus, there can be provided a thin film capacitor formed on the thin flexible substrate, because a highly elastic electrode can assist to relieve the stress caused when dielectric thin film is formed at a temperature higher than a curing temperature of a resin used for a substrate. As a result, it can be possible to provide a thin and high-capacity thin film capacitor with high reliability and to prevent the dielectric thin film itself from being damaged.
In addition, in order to attain the objects, according to the invention, there is provided a complex passive component characterized in that it has, formed on a same substrate, a thin film capacitor and either one or both of a thin film resistor and an inductor, the thin film capacitor having a lower electrode, a dielectric thin film and an upper electrode stacked on a substrate having a thickness equal to or more than 2 xcexcm and equal to or less than 100 xcexcm, and being characterized in that the lower electrode includes a first adhesion electrode in contact with the substrate, an anti-oxidation electrode in contact with the dielectric thin film and a highly elastic electrode made of a material having a Young""s modulus higher than that of the anti-oxidation electrode and provided between the first adhesion electrode and the anti-oxidation electrode.
Furthermore, according to another aspect of the present invention, a thin film capacitor manufacturing method comprises forming a lower electrode on a substrate including forming a highly elastic electrode on the substrate and forming an anti-oxidation electrode on the highly elastic electrode, forming a dielectric thin film on the lower electrode, and forming a upper electrode on the dielectric thin film.
Furthermore, according to another aspect of the present invention, a printed circuit board incorporating a thin film capacitor according to the present invention, wherein the thin film capacitor is embedded in a resin substrate, the first lower electrode is connected electrically via any of a via hole or a through hole or a conductive pattern, and the first upper electrode is connected electrically via any of via hole or a through hole.
According to the printed circuit board of the present invention, a flexible substrate incorporating the high-capacity capacitor can be provided, because it can prevent the thin film capacitor from being damaged when it is incorporated in the printed circuit board. In addition, a printed circuit board incorporating the high-capacity capacitor can be thinned. In addition, since the thin film capacitor can be formed on the flexible substrate, the thin film capacitor can be prevented from being damaged when it is incorporated in the printed circuit board. In addition, since the thin film capacitor according to the invention can have a interconnecting pad electrode having a thickness of 1 xcexcm or more formed therein, through-hole connection with a low resistance and high reliability which is not damaged by a thermal cycle can be realized.